MECHANICAL TESTS

FOR POLYMERSVito Gigante, Laura Aliotta, Luca Panariello, Maria-Beatrice Coltelli, Andrea Lazzeri

WHY TESTING POLYMERS?

❑ Identify the characteristics, properties and

flaws.

❑ Comply with all relevant national and

international standards.

❑ Ensure that your materials meet your clients’

safety, environmental and aging requirements

HOW TO TEST POLYMERS?

Knowledge of the properties of materials is essential for several purposes:

design, specification, quality control, failure analysis and for understanding

the structure and behaviour of new materials

❑ Specific test procedures have been developed

for each type of property to be measured

❑ These procedures are generally those found

best suited to the generic characteristics of the

material class

❑ Standard tests help to provide the most

meaningful results and to allow comparison of

data from different sources

PURPOSE OF TESTING

Approach differences

❑ For quality control, the test should preferably be as simple, rapid and inexpensive as possible. Non-destructive methods

and automation may be particularly attractive. The best tests will additionally relate to product performance.

❑ For predicting product performance the more relevant the test to service conditions the more satisfactory it is likely to be.

Extreme speed and cheapness are less likely to be important but there is a need for test routines which are not

excessively complex. Non-destructive methods may be acceptable.

❑ For producing design data, the need is for tests that give material property data in such a form that they can be applied

with confidence to a variety of configurations. This implies very considerable understanding of the way material properties

vary with geometry, time etc. Extreme speed and cheapness are of relatively minor importance, there is little interest in

non-destructive methods. For complex and long running tests, automation may be desirable.

❑ For investigating failures the first difficulty is to establish what to look for and then the primary need is for a test which

discriminates well (highly sensitive). There is often little need for absolute accuracy or, in some cases, even relevance to

service.

MAIN PROPERTIES TO

EVALUATE - Outline

❑ Tensile

❑ Flexural

❑ Impact

❑ Fracture Toughness

❑ Tearing

❑ Time/Temperature dependant

measurements (HDT, transitions

evaluated with DMTA)

BEFORE THE TEST - HOW TO OBTAIN SAMPLES?

❑ The properties of a material, and

hence the test results, are

dependent on how the material

was formed, not only on whether

there was any cutting or

machining involved but on the

details of mould configuration

and moulding conditions.

❑ Consequently, knowing

exactly how a test

piece was formed is

essential information

for understanding the

significance of the

results

❑ For results to be

comparable it is

essential that a

consistent procedure is

used for the test piece

production.

❑ What kind of “sampling” is better? The

debate is open. Probably to obtain

material properties for use in a data

sheet the preferred approach will be to

mould using standardised procedures

and conditions. For investigations relating

to a product the conditions relevant to

production may be of more interest.

TENSILE TESTS

❑ The short-term tensile characteristics of a material are probably

the most commonly considered of all the properties that can be

determined. Although there are many standards relating to short-

term tensile testing, they all endeavor to quantify a number of

specific characteristics which relate to the strength and

deformation of a material.

❑ Tensile stress-strain characteristics are derived by monitoring both

the force required to pull a material apart and the displacement

that the material undergoes as a result of the applied force at a

constant deformation rate.

TENSILE TESTS - Apparatus & Samples

❑ ISO 527-2 defines specimen to be used for tensile

tests. Types 1A and 1B are standard specimens for

comparable data.

Grips

Load

Cell

Extensometer

❑ Until the relationship between stress and strain is linear up to the failure point and as a consequence the relationship stress/

strain = constant. This constant is known as the elastic modulus of the material and is usually measured in GPa.

❑ Modulus measurement requires for a highly accurate extensometer.

❑ The tensile modulus has to be calculated between two strains, more precisely between 0.05% and 0.25% strain.

❑ It can be calculated as a secant between 2 points or by a linear regression calculation.

Evaluation of Elastic Modulus

Typical stress-strain

curves for polymers

❑ Curve a: Brittle materials

❑ Curve b and c: Tough materials with

yield point

❑ Curve d: Rubber-like behaviour

Properties observed:

❑ Tensile-Modulus

❑ Yielding

❑ Break points

❑ These curves are referred to the

starting section of the sample, this

means that they are the “engineering

stresses and strains”

...more in detail!

Polymers like PLA, PHB,

exhibit type a curves.

Properties allowed to

evaluate:

❑ Tensile modulus

❑ max stress

❑ max strain

Polymers like PBS, exhibit type

b curves:

❑ Modulus, Et

❑ Yield stress, σy

❑ Yield strain, εy

❑ Max stress, σM

❑ Strain at max. stress, εM

❑ Stress at break, σB

❑ Nominal strain at break, εB

Polymers like PLA/PBAT

blends, exhibit type c curves.

Modulus, Et

❑ Yield stress, σy

❑ Yield strain, εy

❑ Max stress, σM

❑ Strain at max. stress, εM

❑ Stress at break, σB

❑ Nominal strain at break,

εB

Polymers like PBAT, exhibit type

D curves. Properties allowed to

evaluate:

❑ Modulus with the elasticity

rubber theory

❑ max stress

❑ max strain

Speed test and sample aspect

❑ Modulus determination has to

be done at lower speed than

the breakage evaluation.

❑ Typical speeds are 1 mm/min

for Modulus, 5 or 50 mm/min

for yielding and stress at break

evaluation.

❑ Tensile modulus and further

tests results can be

determined for at least 5

specimens (reproducibility has

to be guaranteed).

❑ It is preferable to unload the

specimen before testing at a

different speed (at least five

tests for Modulus and five for

the breakage).

❑ The necking appearance is typical of ductile polymers in which both upper and lower

yield points are evident on the curve, which are followed by a near horizontal region.

❑ At the upper yield point, a small neck forms within the gauge section of the specimen.

Within this neck, the chains become oriented (i.e., chain axes become aligned parallel

to the elongation direction) which leads to localized strengthening.

❑ Specimen elongation proceeds by the propagation of this neck region along the gauge

length; the chain orientation phenomenon accompanies this neck extension. This

tensile behavior may be contrasted to that found for ductile metals wherein once a

neck has formed, all subsequent deformation is confined to within the neck region.

Necking development and rate/temperature effect

❑ The mechanical characteristics of polymers are much more sensitive to temperature

changes near room temperature

❑ It should be noted that increasing the temperature produces a decrease in elastic

modulus, a reduction in tensile strength, and an enhancement of ductility. While there

is considerable plastic deformation at both 50 and 60°C.

❑ The influence of strain rate on the mechanical behavior may also be important. In

general, decreasing the rate of deformation has the same influence on the stress–

strain characteristics as increasing the temperature; that is, the material becomes

softer and more ductile.

❑ Flexural stress-strain characteristics are derived by monitoring both the

force required to flex a material and the displacement that the material

undergoes as a result of the applied force at a constant deformation rate.

❑ Flexural tests also have the advantage that a strip test piece is easier to

produce than a dumbbell and there are no gripping problems as can occur

in tensile tests.

❑ The mode of loading can take one of three forms:

❑ Three point

❑ Four point

❑ Simple cantilever

❑ By far the most common is three point loading. As the name implies, this

mode of loading is achieved by applying the force to the specimen at three

points

FLEXURAL PROPERTIES

ASTM D790 - Standard Test Methods for

Flexural Properties of Unreinforced and

Reinforced Plastics

EN ISO 178 - Plastics. Determination of

flexural properties

Common

Standards

3-point

configuration

Flexural

Stress &

Strain

Flexural

Modulus

Flexural stress (σf) = 3Fl/2bh2

Flexural strain (εf) = 6hs/l2

where:l = Support span - the length of the beam between

the centres of the two

outer supporting rods (mm)

h = The thickness of the beam (mm)

b = The width of the beam (mm)

F = Force (N)

s = Deflection of the specimen at mid span (mm)

σf = Flexural stress (N mm-2)

εf = Flexural strain

3-point FLEXURAL PROPERTIES

where slope is the slope of force-deflection

curve between reference strains (0.05% and

0.25% in ISO 178).

Speed & Sample dimensions

❑ A minimum of five test pieces are required by ISO 178

for each direction tested.

❑ The preferred test piece is a strip with the following

dimensions:

❑ Length: l = 80.0 ± 2.0

❑ Width: b = 10.0 ± 0.2

❑ Thickness: h = 4.0 ±

❑ The thickness of the central third of the specimen

length shall not deviate by more than 2 %

❑ The width of the central third of the specimen length

shall not deviate by more than 3%.

❑ The specimen must have a rectangular cross section

with no rounded edges.

❑ The length to thickness ratio shall be 20 (l/h = 20 ± 1)

IMPACT PROPERTIES

❑ Impact properties of plastics

materials are directly related to

the overall toughness of the

material. The concept of

‘toughness’ is the work done in

breaking a test piece or object.

❑ The Impact test is a

standardized high strain-rate

test which determines the

amount of energy absorbed by

a material during fracture.

❑ On molded samples the test is

performed by striking a notched

specimen with a moving mass

(a hammer). On films is

preferred the falling dart

method

❑ The energy absorbed can be

evaluated in Charpy or Izod

configurations (more details in the

following slides), through a

load/displacement curve or

correlated with the tensile

properties on samples .

Charpy Impact test

❑ Standards: ASTM D6110, ISO 179 The test

can be done on notched (V-shape or U-

shape) or unnotched samples

M

hs

hf

ΔE = M*g*(hf-hs)

❑ WHAT IS OBTAINED FROM THE TEST? It is

a balance of mechanical energy! The energy

absorbed by the specimen is proportional to

the difference in height.

❑ Impact resistance is defined as the ability of a

material to absorb energy resulting from a

collision:

Impact strength = IS = ∆𝑬

𝑺𝟎

❑ ΔE is the absorbed energy and S0 is the

resistant section of the specimen (the part not

notched).

❑ Example of Charpy

impact tests. ISO

179 v-notched

samples 80x10x4

mm with 2mm 45°

notch.

Charpy Impact test

3

14

2

1. Control system

2. Pendulum with a final

hummer

3. A fulcrum supported for the

release and the brake of the

hummer

4. Two-point support for the

specimen for the Charpy

configuration

❑ The Izod test is notionally very similar to the Charpy test, except that the test piece is clamped at one end just below

the notch, or the centre of the specimen if it is unnotched, and struck by a pendulum close to the other end.

❑ The standard used for these tests are: EN ISO 180 and ASTM D256.

❑ Unlike the Charpy test, the notched Izod is capable of being tested either with the notch on the same side as the point

of impact, which is the normal way round, or on the opposite side when it is called the reverse notched test. Thus, in

the normal test the side containing the notch is placed under tension and the notch fulfils its purpose as a stress

concentrator.

❑ The hammer swings downward, hits the test material in the middle, at the bottom of its swing, and then is left free at the

top.

❑ It is not applicable to compound materials because of the influence of complicated and inconsistent failure modes.

IZOD test

❑ Energy is required both to create a crack and to allow this

crack to be propagated through the material. The energy to

initiate a crack is called the crack initiation energy. If the

available energy in the system undergoing impact exceeds

the crack initiation energy, the crack will continue to

propagate, and complete failure will occur if the system has

sufficient energy to also exceed the crack propagation

energy.

❑ Four basic types of failure that are encountered under

impact and the result of an impact test may result in

different types of failure:

Fracture types

❑ The distinction between the four types of failures is not

always very clear and some overlapping is quite possible.

❑ For Charpy and Izod tests, for example, the following

definitions along with their letter abbreviations need to be

adopted:

❑ Brittle fracture is where the part fractures

extensively without yielding and typically has sharp

‘glassy’ edges.

❑ Ductile failure is where there is a definite yielding of

material, often indicated by stress whitening, along

with cracking.

❑ C complete break; a break in which the

specimen separates into two or more pieces.

❑ H hinge break; an incomplete break such that

both parts of the specimen are held together

only by a thin peripheral layer in the form of a

hinge having no residual stiffness.

❑ P partial break; an incomplete break that does

not meet the definition for a hinge break.

❑ NB non-break; in the case where there is no

break, and the specimen is only bent, possibly

combined with stress whitening.

Factors that influence fracture

01 02 03Rate of Loading

❑ Low rates of impact,

relatively stiff materials can

still have good impact

strength, but at high enough

rates of impact, even

rubbery materials will exhibit

brittle failure.

❑ All polymer materials seem

to have a critical velocity

above which they behave as

glassy, brittle materials.

Temperature

❑ Decreasing the temperature

tends to promote the onset of

brittle failure.

❑ Note that increasing

temperature has the opposite

effect of increasing speed and

so there is not a single

temperature at which brittleness

occurs, but a locus of

temperature/speed values

where the transition from ductile

to brittle behaviour takes place.

Notch Sensitivity

❑ A sharp corner in a test

specimen can

dramatically lower the

impact strength of the

material.

❑ A notch creates a

localized stress

concentration where the

true stress can be many

times higher than the bulk

stress being imposed on

the test piece or object as

a whole.

Other Impact tests: Instrumented Impact test and falling dart

❑ ISO 17282 at 1m/s on damped notched samples to obtain a load/displacement curve starting from a Charpy Impact test

❑ Falling dart method: films and sheets are

tested in this way, typically 125 mm, as

does the impacting striker. The

standardized tests are given in ISO 7765.

❑ The dart with a 20 mm diameter striker is

released from a preferred height of one

meter. The test piece may be clamped or

unclamped on the support

❑ All the traditional standard stress-

strain tests for plastics have some

limitations because the results are

geometry dependent and they do

not yield fundamental properties.

❑ Fracture mechanics provides a way

of interpreting the material response

independently of geometry through

consideration of the loads or

stresses that cause a crack to

propagate.

❑ The two types of fracture

mechanics, depending on the

fracture behavior of the polymers,

are LEFM and EPFM.

Fracture toughness

Fracture toughness -LEFM

❑ Linear Elastic Fracture Mechanics (LEFM) is the basic theory of fracture, that deals with sharp cracks in elastic bodies. It

describes the energy change which occurs when such a body undergoes an increase in crack area.

❑ Fracture mechanics study starts from three-point bending test on SENB specimens.

❑ Properties evaluated: in terms of stress intensity, the critical stress intensity factor, Kc,

is the minimum stress intensity for fracture to occur and, although called a factor, has

units of Pa m0.5.

❑ Gc is the Elastic energy release rate and it is related to Kc through the Elastic Modulus.

❑ Instead to relate Gc and Kc to the measured load or energy requires a calibration factor

which is a function of the crack length and the test piece width (the geometry of the

sample).

Fracture toughness - EPFM.

❑ Elastic-Plastic Fracture Mechanics (EPFM) represents the study of a stable

crack growth and the presence of a plastic zone at the crack tip. The energy

release rate is the Jc , that is the equivalent of Gc for a non-linear elastic

material.

❑ Procedure to evaluate Jc:

❑ Three point bending on SENB samples at different crack growth

❑ Microscopic analysis to evaluate Δa

❑ The energy is measured and Jc determined to give an 'R' curve that is a

power law curve:

❑ The true initiation value of crack propagation is determined when this

line intercepted the crack growth curve:

❑ ISO 6133 states that the two ‘legs’ are gripped in the stationary

and moving grips of a universal testing machine and pulled

apart at 200 or 250 mm/min.

❑ Typically, an irregular wavelike trace results and the standard

defines the tearing force as the mean force after ignoring the

first 20 mm and last 5 mm of the tearing trace. This tearing

force is then normalized by dividing it by the film or sheet

thickness to produce the tearing resistance value.

TEAR PROPERTIES

❑ A common technique to measure the critical fracture energy during fracture of films is trouser tear test. The origin of the

name is correlated to the trouser shape of the samples. The legs of the trouser are pulled in opposite direction to create

the tearing action

TestingCutting

Correctdimensions

Machine & Cross

DirectionSamples

❑ Critical Fracture Energy (N/m) is calculated in two different ways if the

legs are stretched (a) or not (b) during the test. (F is the load, 𝜆 the ratio

lenght, t is the thickness, w is the width, E is the Energy density.)

a) 𝑇 =2λ𝐹

𝑡− 𝑤𝐸

b) 𝑇 =2𝐹

𝑡

TEAR TESTING

❑ HDT is defined as the temperature at which a standard test

bar (ISO 75 - 80x10x4 mm or ASTM D648 - 127x13x3mm)

deflects 0.025 mm under a stated load of either 0.455 or 1.82

MPa.

❑ HDT values are used to compare the elevated temperature

performance of the materials under load at the stated

conditions.

❑ The result is the temperature at which a specified deformation

or penetration is achieved. Four replicate specimens are used

for each test.

❑ Conditioning: 23 ± 2°C and 50 ± 5% RH for not less than 40

hours prior to test.

HEAT DEFLECTION TEMPERATURE

❑ The weight of the rod used to transfer the force on the test specimen is

included as part of the total load.

❑ The load (P) is calculated as: P = 2Sbd2/3, where:

❑ S = Max. stress in the specimen of 0.455 MPa or 1.82 MPa

❑ b = Width of specimen

❑ d = Depth of specimen

❑ L = Width of span between support

❑ The temperature of the medium is measured when the test bar has

deflected 0.25mm (0.010 in).

❑ This temperature is recorded as the deflection temperature under

flexural load of the test specimen.

HDT Apparatus & Procedure

DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA)

❑ Dynamic Mechanical Thermal

Analysis (DMTA) records material’s

temperature-dependent visco-elastic

properties and determines its

properties by applying an oscillating

force to the sample.

❑ DMTA measures stiffness and damping,

these are reported as modulus and tan delta.

Because of sinusoidal stress is applied,

modulus can be expresesed as in-phase

component, the storage modulus (E‘) and

out of phase component, the loss modulus

(E").

❑ Storage modulus (E’) is a measure of elastic

response of a material. It measures the

stored energy. Being in-phase with the

applied stress, represents the elastic

component of the material’s behaviour

❑ Loss modulus (E") is a measure of viscous

response of a material. It measures the

energy dissipated as heat. It corresponds to

the viscous nature of the material.

❑ Tan delta (tan ) is the ratio between

loss modulus and storage modulus and

it is called damping. It is a measure of

the energy dissipation of a material and

should range between 0° and 90°.

❑ When tan delta is around 0° the

material behaviour is purely

elastic. Otherwise when tan delta

approaches 90°, the material behaviour

is purely viscous.

❑ Compression

❑ Tension

❑ Shear

❑ Torsion

DMTA TYPE OF TESTING

Frequency SweepLo

g E’

,E’’

Frequency

Temperature Sweep

Log

E’,E

’’

Temperature

---E’’E’

❑ Why do E’ and E” vary with frequency and temperature? The chain can alter itsconformation and its entanglements relative to the frequency (or at the temperature) ofthe load.

❑ DMTA analysis can be realized changingfrequency or temperature. In any case four typicalzones can be evaluated for polymers. DMTAgraphs are useful to evaluate polymer transitions.

1. Terminal Zone: Period of oscillation is so long that chains cansnake through their entanglement constraints and completelyrearrange their conformations.

2. Plateau Zone: Strain is accommodated by entropic changes topolymer segments between entanglements, providing goodelastic response.

3. Transition Zone: The periodof oscillation is becoming tooshort to allow for completerearrangement of chainconformation. Enough mobilityis present for substantial frictionbetween chain segments.

4. Glassy Zone: Noconfigurational rearrangementsoccur within the period ofoscillation. Stress response to agiven strain is high (glass-likesolid).

❑ Resistance: ability to resist to an applied force without

fracturing.

❑ Elasticity: the capacity of a material to deform under an

applied load and when it ceases it returns to its original

dimensions.

❑ Plasticity: ability to maintain the deformation imposed.

❑ Ductility: ability of a material to be reduced into wires.

❑ Inelasticity: time-dependent elastic deformation.

❑ Creep: ability of a material to deform permanently. The

deformation degree depends on the load application time

and temperature.

❑ Brittleness: Easiness of a material to break at the

minimum shock. The break occurs with a crash without

warning.

❑ Toughness: ability to resist to a fracture undergoing a

deformation.

❑ Hardness: resistance to abrasion and indentation and,

more in general, to a plastic deformation.

BEFORE TO FINISH…A GLOSSARY OF MECHANICAL PROPERTIES

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